1. Field of the Invention
The present invention relates to a method and to a system for remote sensing: of the flammability of the different parts of an area flown over by an aircraft in order to facilitate preventive actions in the most threatened parts.
2. Description of the Prior Art
Fire hazards that can affect a vegetal area depend on many factors. Some factors among the major ones are:
1) the structure of the plant cover, the presence of composite dead plants being a favoring factor according to the density thereof;
2) the botanic composition of the plant cover, because certain vegetal species are more vulnerable than others, brushwoods and dead plants for example are more flammable than timber trees, certain tree species such as coniferous trees for example are more flammable than others. The study of this factor involves an analysis of the plant cover maps, followed by a photographic survey allowing the analysis to be refined;
3) the orientation of the slopes on which the vegetation grows, the slopes getting the most sunshine being the most vulnerable to the action of the fire. A digital terrain model (DTM) of the area studied is generally used to take account of this second risk factor; or
4) the hydric deficit of the soil indicating a hydric stress of the vegetation, which decreases the natural ability of plants to regulate their temperature through evaporation.
Detection of hot spots at the ground surface by remote sensing is a relatively old technique. Various studies relating to phenomena linked with fires which are detectable by remote sensing, to the use of radiation in the thermal band and image processing methodologies are described for example in the following documents:
Hirsch S. N. et al., 1973, The Bispectral Forest Fire Detection System, in The Surveillant Science, Holz Ed., Houghton Mifflin Cy, Boston; PA1 Goillot C. et al., 1988, Etude Dynamique des Feux de Forets par Scanner Aeroporte Multibande dans le Visible et le Thermique, in Proceedings ISPRS, Kyoto; PA1 Leckie D. G., 1994, Possible Airborne Sensor, Processing and Interpretation Systems for Major Forestry Applications, in Proceedings of the first International Airborne Remote Sensing Conference and Exhibition (I.A.R.S.C.E.), Strasbourg; or PA1 Ambrosia V. G. et al., AIRDAS, 1994 Proceedings of the I.A.R.S.C.E., Strasbourg. PA1 Che N. et al., Survey or Radiometric Calibration Results and Methods for Visible and Near Infrared Channels of NOAA-7, -9 and -11 AVHRRs, in Remote Sens. (1992).
It is well-known to combine signals corresponding to radiations emanating from a surface element on the ground, in the red part of the spectrum (0.6 .mu.m&lt;.lambda..sub.1 &lt;0.7 .mu.m for example) and the near infrared (0.8 .mu.m&lt;.lambda..sub.1 &lt;1.1 .mu.m for example), which allows, after normalization, the obtaining of the state of "hydric stress" of vegetable matter i.e. to know if it has enough water resources to compensate for the evaporation corresponding to the ambient temperature. Such a combination used aboard a satellite is described for example in:
Various techniques implementing fire remote sensing are also described in French Patents 2,224,818, 2,614,984, and 2,643,173, European Patents 490,722 and 611,242, and WO-93/02,749.
In regions where chronic fire hazards are high, mainly during the warm season, in their concern for good management of the national heritage, have installed ground or airborne detection systems allowing early alert of the fire-fighting forces and allowing analysis of the various parameters characteristic of the fire that has broken out and for following the spread thereof.
Fighting a fire is generally more effective if it is possible to foresee or to predict how it is likely to break out and to spread, so as to start preventive actions such as surface watering in areas that appear to be the most threatened after analysis.